What keeps friendships alive?

In search for the keys to enduring friendships, a physicist and sociologist from Notre Dame examined more than 8 million phone calls between 2 million people.

Their answer, as the science news site PhysOrg admits, may not surprise you: “the leading cause of persistent relationships is reciprocity — returning a friend’s call.” Further, they said friends ’til the end tend to touch base at least once every 15 days.